1. Field of the Invention
The present invention relates to an image forming device such as a color laser printer.
2. Description of the Related Art
A tandem laser printer is one type of laser printer for forming full-color images. One example of a tandem laser printer includes an image forming unit for each color. Each image forming unit includes a developing roller, a photosensitive drum, a charge unit, and an exposure unit. The developing roller, the charge unit, and the exposure unit are disposed in confrontation with the photosensitive drum. The charge unit forms a uniform charge on the surface of the photosensitive drum. The exposure unit selectively exposes portions of the charged surface to form a latent static-electric image on the surface of the photosensitive drum. The developing roller bears toner on its surface and develops the latent static-electric image using the toner.
The visible toner images developed for each color are transferred one at a time in order onto a transfer belt so that a full-color image can be formed at substantially the same speed as a monochrome image.
The photosensitive drums of this tandem laser printer normally include a drum body and support members. The drum body has a hollow tubular shape. The support members are press fitted into the inner periphery surface of the drum body from the axial ends of the drum body. The support member includes a support shaft that is freely rotatably supported, for example by the frame of the image forming device, so that the photosensitive drum is in turn rotatably supported.
However, with this configuration the photosensitive drum will rotate eccentrically if the support shaft is shifted from the circumferential center of the photosensitive drum. When the photosensitive drum rotates eccentrically, the outer peripheral surface of the photosensitive drum will contact the transfer belt with varying peripheral speed. The different color toner images can be shifted out of place with respect to each other because the outer peripheral surface of the photosensitive drum speeds up and slows down in this manner. For example, if a visual image made from one color of toner is transferred to the transfer belt while the outer peripheral surface of the photosensitive drum moves at a high speed and then a visual toner image made from another color of toner is transferred onto the first color image while the outer peripheral surface of the photosensitive drum moves at a slow speed, then the first color toner image will be shifted from the second color toner image. The images will be shifted from each other if the first image is transferred while the peripheral speed is slow and the second image is transferred while the peripheral speed is fast.
Therefore, the support member must be formed with high precision. However, even if the support member is formed with high precision, the photosensitive drum will rotate eccentrically if the inner peripheral surface of the photosensitive drum is made with poor precision. Unlike the outer peripheral surface of the photosensitive drum, which needs to be formed with extremely high precision in order to form the latent-static-electric images properly on the surface of the photosensitive drum, the inner peripheral surface of the photosensitive drum does not need to be made with high precision. Because the inner peripheral surface of the photosensitive drum is made with poor precision, the support shaft of the support member can be shifted from the circumferential center of the photosensitive drum when the support member is press inserted into the photosensitive drum, so that the photosensitive drum will rotate eccentrically.
On the other hand, if the inner peripheral surface of the photosensitive drum is formed with high precision in order to prevent such shifts between different color images, then the cost of the image forming device will increase greatly.
It is conceivable that the tandem laser printer be designed so that the transfer belt is driven by movement of the photosensitive drums. In this case, the toner images would be transferred onto the transfer belt at the peripheral speed where the photosensitive drums contact the transfer belt. However, if there is variation in the drive force produced by the difference photosensitive drums for driving the transfer belt, then a larger burden will be placed on some photosensitive drums than on others in order to move the transfer belt. As a result, the photosensitive drums would have different peripheral speeds at the exposure position, resulting in latent-static-electric images being defectively formed.
It is an objective of the present invention to overcome the above-described problems and provide an image forming device capable of simply and reliably preventing eccentric rotation of the photosensitive drums so that images can be formed properly.
An image forming device including a photosensitive body and a support. The support supports the photosensitive body in a rotatable condition by supporting the outer peripheral surface of the photosensitive body. With this configuration, the support member supports the photosensitive body at the outer peripheral surface of the photosensitive body. Because the outer peripheral surface of the photosensitive body is formed with extremely high precision, this configuration insures that the photosensitive body will rotate with high precision and without any eccentricity. For this reason, visible images formed on the photosensitive body will transferred from the photosensitive body at a uniform speed. Therefore, eccentric rotation of the photosensitive body can be simply and reliably prevented so that proper images can be formed.